You've obviously been watching too many TV shows. Shooting at the extremities of a person is not a sure thing. It's easier to be moving your hand around faster than the center of your mass. And what happens if the perp moves their hand suddenly and your shot misses? Do you think the OTHER person with a gun would start firing back, hitting who knows what(or whom)?
That is simply a bad idea.
I did watch this on a "Reality TV Show" where a suicidal man with a gun had it shot out of his hand by a Police sniper in America.